1. Field of the Invention
The present invention generally relates to bandwidth updating methods and bandwidth updating apparatuses and, more particularly, to a bandwidth updating method and a bandwidth updating apparatus for a communications system of time-division multiple access type in which a plurality of subscriber apparatuses are connected to a single transmission channel.
2. Description of the Related Art
One example of a communications system of a time-division multiple access type in which a plurality of subscriber apparatuses are connected to a single transmission channel is an Asynchronous Transfer Mode-Passive Optical Network (ATM-PON) system described in the ITU-T Recommendation G.983.1. FIG. 7 shows a construction of an ATM-PON system. Referring to FIG. 7, the system comprises a station apparatus 1 provided at a station, subscriber apparatuses 2-1–2-N provided at the subscribers' end and capable of bidirectional communication with the station apparatus 1, a transmission channel 3 shared by the station apparatus and the subscriber apparatuses 2-1–2-N, a transmission enable information generating function 11, a downstream signal generating function 12, a physical interface function 13, a upstream signal terminating function 14, and a bandwidth control function 15.
Downstream communication from the station apparatus 1 to the subscriber apparatuses 2-1–2-N is broadcast of the same data to the subscriber apparatuses 2-1–2-N. Each of the subscriber apparatus retrieves data destined for the retrieving apparatus from the received data. In upstream communication from the subscriber apparatuses 2-1–2-N to the station apparatus 1, the subscriber apparatuses 2-1–2-N are allowed to transmit upstream signals only in those time slots allocated by the station apparatus 1 to the transmitting apparatus. Transmission enable information designating the time slots in which the upstream signal transmission is enabled is generated by the transmission enable information generating function 11 and multiplexed by the downstream signal generating function 12. The size per unit time designated by the transmission enable information provided to the subscriber apparatus is based on a bandwidth allocated to the subscriber apparatus in accordance with a communications service agreement. The bandwidth control function calculates the size per unit time designated by the transmission enable information transmitted to the subscriber apparatuses 2-1–2-N and sets the size in the transmission enable information generating function 11, in accordance with the number of connections accommodated in the subscriber apparatuses 2-1–2-N and variation in capacity required in the connections.
In the bandwidth updating system according to the related art, the transmission enable information should be given to the subscriber apparatuses 2-1–2-N so as to ensure that the bandwidth adapted for a peak cell rate (PCR) is available even when a variable bit rate connection such as a VBR (variable bit rate) connection and a UBR (unspecified bit rate) connection is set up. Since a fixed bandwidth is allocated to each of a plurality of subscriber apparatuses, the bandwidth allocated to a unit subscriber is small. Another disadvantage is that, since the fixed bandwidth continues to be allocated even when the traffic is small with respect to the allocated bandwidth, the efficiency of bandwidth usage is relatively low.